In terms of vocal range it is not that unusual to vary our speech by an octave of more. When you are relaxed you might be in a lower register, but then if you are suddenly excited going an octave higher can happen from the constriction in the throat that happens involuntarily. I would say you would be reasonably safe to write a song that may extend up to an octave and a half without really challenging too many singers. They may need to transpose a key, but the range would not be a huge challenge. That is if they really sing. On the other hand, I would be willing to bet that the vast majority of pop tunes do not exceed an octave in range.

Jack
http://www.myspace.com/jackswainsongs